This invention pertains to the treatment of industrial process water, and more particularly, to the removal of bromine based biocides in industrial process water streams. Specifically, the invention relates to the removal of bromine based biocides from industrial process water streams using ultraviolet (UV) radiation.
A wide variety of industries use water as the coolant for heat transfer processes. A significant quantity of cooling water is used annually by both electrical power plants and manufacturing operations. Typical water cooled heat transfer processes include condensers and chillers. In general, the spent water is recycled to lessen both the economic and environmental impact of these heat transfer processes. This recycling requires the spent water to be cooled back to ambient temperature or slightly below, commonly by means such as a cooling tower. In general, cooling towers allow the spent water to release heat to the ambient air by cascading the spent water down an open air tower.
Cooling water, commonly supplied by a surface water source such as a lake or stream, contains a wide range of contaminants. These contaminants can lead to fouling and subsequent reduction of coolant flow and/or heat transfer efficiency. Minerals, such as calcium, magnesium and silica, can form scale deposits on the insides of pipes and tubes. Bacteria, carried into the cooling water system by wind or make up water, can likewise foul process piping and heat exchange surfaces by creating deposits, or, if left unchecked, a continuous layer of surface slime. Bacterial contamination is particularly troublesome because the heat transfer through heat exchanger surfaces can be significantly reduced, corrosion can occur under the bacterial slime layer, and the tacky nature of the slime layer provides a surface on which minerals can readily deposit.
To control the build up of these contaminants, industries typically subject at least a portion of their process water stream, commonly referred to as a xe2x80x9cblowdown stream, to a purification system. In general, process water purification systems are comprised of a series of removal processes, each designed to eliminate specific types of contaminants. For example, minerals may be removed from process water streams by methods such as deionization and reverse osmosis. The presence and growth of microorganisms may be controlled using methods such as oxidation, ultraviolet irradiation, and mechanical cleaning. Oxidation, the traditional method by which to eliminate microorganisms, remains popular today. Oxidizing biocides, such as chlorine and bromine, kill microbial growth by destroying important cellular components within the microorganisms.
Chlorine, first used to purify water in the 1800s, remains the most widely used oxidizing biocide. Chlorine provides its biocidal properties by reacting with water to form hypochlorous acid, a very strong oxidizing agent, in a pH sensitive reaction. However, although chlorine is an effective, easily applied biocide, it has several drawbacks. In 1982 the U.S. Environmental Protection Agency implemented regulations which effectively limited the use and the discharge of chlorine. Further, high concentrations of halogenated biocides, such as those required with chlorine biocides, are known to increase the corrosion of carbon steel. Chlorine is particularly known to corrode copper alloys, widely used in heat exchange equipment. Chlorine is also known to degrade the wood fill used in cooling towers, leading manufacturers to use plastic fill. Further, chlorine is known to form undesirable reaction products with organics and ammonia compounds commonly found in process water streams.
Bromine, introduced as a process water biocide in the 1940""s, is an attractive alternative to chlorine. Bromine biocide may be applied to process water in several forms, including as a liquid solution of pure bromine, as a bromine/chlorine mixture, or as a bromide-surfactant package. Recent product advances have resulted in biocide packages containing bromine granules with an equivalent molar ratio of chlorine for quick activation and easy handling. The chlorine is included in the biocide package to react with the bromine in water solution, thus producing hypobromous acid and an innocuous chloride ion via the following nonreversible reaction:
xe2x80x83HOCl+Brxe2x88x92xe2x86x92HOBr+Clxe2x88x92xe2x80x83xe2x80x83(1)
The chloride ion subsequently bonds with available cations in the water solution, such as sodium, to form salts, such as sodium chloride. The hypobromous acid which is formed is a strong oxidizing agent. Hypobromous acid is the active biocide formed by bromine, much as hypochlorous acid is responsible for chlorine""s biocidal properties. However, in comparison, bromine dissociates into hypobromous acid much more effectively than chlorine dissociates into hypochlorous acid at higher pHs. In addition, the subsequent biocidal reaction of hypobromous acid is also highly effective at elevated pHs. The biocidal reaction of hypobromous acid in water is:
HOBr+R3CHxe2x86x92R3COH+H++Brxe2x88x92xe2x80x83xe2x80x83(2)
In cooling towers in which the pH has been elevated with a caustic agent, such as NaOH, the hydronium ion (H+) and bromide ion (Brxe2x88x92) formed in the biocidal reaction (2) combine with the dissociated ions formed by the caustic agent, such as Na+ and OHxe2x88x92, to form a bromide salt, such as Na+Brxe2x88x92, and water, H2O. These reaction products, i.e., bromide salt and water, are favored, thus driving the biocide reaction (2) to completion. The superior dissociative properties and biocidal activity of bromine is particularly important in applications in which cooling water is supplied by surface waters, which tend naturally to have higher pHs and high nutrient levels for biogrowth.
In addition to greater oxidative efficacy at higher pHs, bromine offers several other significant advantages over chlorine in process water purification, including reduced corrosion to process piping and heat exchanger surfaces and lower vapor pressure. In particular, a lower vapor pressure biocide is advantageous because it reduces the rate of loss of bromine to the atmosphere as the cooling water cascades down through the cooling tower packing.
Although the strong oxidizing power of bromine makes it a highly effective biocide, its oxidative properties have a detrimental impact on other water purification processes downstream. In particular, bromine attacks the deionization resins and reverse osmosis membranes typically used to remove inorganic and organic contaminants. Currently, bromine is removed from process water streams by means of a reducing agent, such as sodium bisulfite. Unfortunately, the use of chemical reagents to remove bromine from a process stream is disadvantageous in that the mechanical system, in particular the pump used to inject the reagent, can potentially fail. Typical pump failures include a mechanical failure of the diaphragm of positive displacement pumps which are typically used, a lack of proper priming on start up after chemical replenishment, power outages, or maintenance down time. Also, injection of a reducing agent is inefficient in purification systems because the compound produced by the reaction of the reducing agent is typically removed downstream by other purification operations. For example, sulfite reaction products, such as those produced by the reaction of sodium bisulfite, must be removed prior to discharging the process water stream back into natural receiving waters, such as a river, lake, or stream. Some aquatic organisms, such as fish, are sensitive to sulfite reaction products, which can impair respiration (oxygen uptake) and liver function. Therefore, there remains a need in the art to convert the bromine biocide used in industrial process water streams into innocuous compounds more reliably and efficiently.
The present invention provides a system and a method by which bromine biocide may be removed from process water streams without the use of either chemical reagents or complex mechanical systems, thus yielding a significantly more reliable, efficient means of removal which, in turn, provides greater protection for downstream purification equipment. In particular, the present invention employs irradiation to decompose, or photodissociate, bromine biocide present in process water effluent streams, thereby forming innocuous compounds. In a preferred embodiment, UV radiation is used to dissociate bromine biocide in a process water stream, thereby yielding innocuous bromide salts.
In one particularly advantageous embodiment, a water purification system for process water containing bromine biocide is provided comprised respectively of an irradiation chamber, a deionizer, and a separation system for removing suspended solids. By irradiating the effluent water stream, compounds less oxidative than the bromine biocide are formed, thereby protecting the downstream deionizer and separation system from degradation. In one aspect of this embodiment, the irradiation chamber is fitted with an UV light source, such as one or more medium pressure mercury vapor lamps. The irradiation chamber may further be comprised of a stainless steel cylindrical member, such as a pipe, through which the effluent stream continuously flows. In its cylindrical embodiment, the working pressure in the irradiation chamber is up to 100 psi, typically 40 psi, the pressure loss is less than 2 psi, and the mercury vapor lamp is rated generally from about 0.5 to about 40 kW, and more advantageously from about 2 to about 5 kW. In one advantageous aspect of this embodiment, a dwell time of 10 to 20 seconds is provided in the cylindrical stainless steel irradiation chamber.
According to one aspect of the invention, the deionizer is a weak acid cationic exchange resin and the separation system is a reverse osmosis system. In a further embodiment, the water purification system also includes a secondary deionizer, such as a sodium zeolite ion exchange resin. In an alternate embodiment, a secondary system is provided for bromine biocide removal, such as a pump for injecting a reducing agent such as sodium bisulfite. Additionally, the water purification system can optionally include a dearator to remove gases trapped in the deionized water. An alternative of this optional embodiment utilizes an alkaline solution pump for elevating the pH of such dearated waters, thereby ensuring that silica is kept in suspension. Typically, the effluent water stream is provided by a cooling tower, and a pumping system is used to introduce the bromine biocide into the cooling tower feed stream. In one aspect that embodiment, only a fraction of cooling tower water, i.e., a side stream, is treated in the water purification system.
Another aspect of the invention is to provide a method by which to remove bromine biocide from a process water stream using irradiation, especially UV irradiation. In one particular embodiment, an effluent water stream laden with bromine biocide is subjected to UV irradiation in an amount sufficient to form bromide anions. In one aspect of the invention, the irradiation process consists of subjecting effluent water containing bromine biocide to UV energy in the range of about 200 mWs/cm2 to 3500 mWs/cm2. The bromine biocide laden effluent water typically contains from 0.1 mg/l to 3.0 mg/l bromine biocide. In one exemplary embodiment, residual bromine biocide is not detectable in the effluent water stream following the irradiation process of the present invention.
By photodissociating bromine biocide as an initial step in a water purification process, the system and method of the present invention protects downstream purification processes from oxidation more reliably and efficiently. Further, the system and method of the present invention are surprisingly robust, capable of treating water used in normal operations, which is generally quite dirty. Additionally, UV based bromine biocide removal also makes the cooling tower blowdown stream more suitable for discharge to natural receiving waters such as a river, lake or stream.
In a particularly advantageous embodiment, photodissociation is accomplished by subjecting an effluent water stream to a sufficient quantity of UV radiation. The dissociated bromine biocide then forms less oxidative compounds, such as bromide salts, by combining with cations naturally occurring in the effluent water stream. These less oxidative compounds do not attack ion exchange resins or reverse osmosis membranes, which may be present downstream. The present invention thus provides a bromine biocide removal technique, which does not involve complex mechanical systems or the introduction of chemicals, which are subsequently removed.
Further understanding of the processes and systems of the invention will be understood with reference to the brief description of the drawings and detailed description which follows herein.